Afrotheria (/æfroʊˈθɪəriə/ from
LatinAfro- "of Africa" + theria "wild beast") is a
superorder of
mammals, the living members of which belong to groups that are either currently living in
Africa or of African origin:
golden moles,
elephant shrews (also known as sengis),
otter shrews,
tenrecs,
aardvarks,
hyraxes,
elephants,
sea cows, and several extinct clades. Most groups of afrotheres share little or no superficial resemblance, and their similarities have only become known in recent times because of genetics and molecular studies. Many afrothere groups are found mostly or exclusively in Africa, reflecting the fact that Africa was an island continent from the
Cretaceous until the early
Miocene around 20 million years ago, when Afro-Arabia collided with Eurasia.
Because Africa was isolated by water,
Laurasian groups of mammals such as
insectivores,
rodents,
lagomorphs,
carnivorans and
ungulates could not reach Africa for much of the early to mid-
Cenozoic. Instead, the niches occupied by those groups on the northern continents were filled by various groups of afrotheres via the process of
convergent evolution. The small insectivorous afrotheres such as elephant shrews, golden moles, and tenrecs filled the niches of
insectivores, the hyraxes filled the roles of rodents and lagomorphs, the aardvarks filled the roles of various medium size ant-eating mammals (anteaters, armadillos, pangolins, echidnas, numbats, etc.) found on other continents throughout the Cenozoic, and
proboscideans (elephants and their relatives) filled the roles of large herbivores such as hippos, camels, rhinos, and tapirs. The
sirenians developed aquatic body plans and started spreading to other parts of the world by water (evolving convergently with the other groups of marine mammals such as
cetaceans and
pinnipeds). In addition to their similarity with Laurasian mammals in North America, Europe, and Asia, many afrotheres also exhibit convergent evolution with groups of mammals that evolved and lived exclusively in South America, which was also an island continent for much of the Cenozoic.
The common ancestry of these animals was not recognized until the late 1990s.[1] Historically, the
Paenungulata had been linked to the true ungulates (particularly the
Perissodactyls); the golden mole, tenrecs, and elephant shrews with the traditional (and polyphyletic/incorrect) taxon
Insectivora; and the aardvarks with the
pangolins and the
xenarthrans within the invalid taxon
Edentata. Continuing work on the molecular[2][3][4] and morphological[5][6][7][8] diversity of afrotherian mammals has provided ever increasing support for their common ancestry.
Evolutionary relationships
The afrotherian clade was originally proposed in 1998[1] based on analyses of
DNA sequence data. However, previous studies had hinted at the close interrelationships among subsets of endemic African mammals; some of these studies date to the 1920s[9] and there were sporadic papers in the 1980s[10] and 1990s.[11][12][13] The core of the Afrotheria consists of the
Paenungulata, i.e.,
elephants,
sea cows, and
hyraxes, a group with a long history among comparative anatomists.[14][15] Hence, while
DNA sequence data have proven essential to infer the existence of the Afrotheria as a whole, and while the
Afroinsectiphilia (
insectivoran-grade afrotheres including
tenrecs,
golden moles,
sengis, and
aardvarks) were not recognized as part of Afrotheria without DNA data, some precedent is found in the comparative anatomical literature for the idea that at least part of this group forms a
clade. The
Paleocene genus Ocepeia, which is the most completely-known Paleocene African mammal and the oldest afrotherian known from a complete skull, shares similarities with both Paenungulata and Afroinsectiphilia, and may help to characterize the ancestral body type of afrotherians.[16]
Since the 1990s, increasing molecular and anatomical data have been applied to the classification of animals. Both types of data support the idea that afrotherian mammals are descended from a single common ancestor to the exclusion of other mammals. On the anatomical side, features shared by most, if not all, afrotheres include high vertebral counts,[8] aspects of
placental membrane formation,[17] the shape of the ankle bones,[6][7] the relatively late eruption of the permanent dentition,[18] and undescended
testicles remaining in the body near the kidneys.[19] The snout is unusually long and mobile in several Afrotherian species, and this was pointed out as a possible shared-derived character.[20] Studies of
genomic data, including millions of aligned
nucleotides sampled for a growing number of placental mammals, also support Afrotheria as a clade.[21][22] Additionally, there might be some dental synapomorphies uniting afroinsectiphilians, if not afrotheres as a whole: p4 talonid and trigonid of similar breadth,
a prominent p4 hypoconid, presence of a P4 metacone and absence of parastyles on M1–2.[7][23]
Afrotheria is now recognized as one of the three major groups within the
Eutheria (containing
placental mammals).[24] Relations within the three cohorts, Afrotheria,
Xenarthra,
Boreoeutheria, and the identity of the placental root, remain somewhat controversial.[5]
Afrotheria as a clade has usually been discussed without a Linnaean rank, but has been assigned the rank of cohort, magnorder, and superorder. One reconstruction, which applies the
molecular clock, proposes that the oldest split occurred between Afrotheria and the other two some 105 million years ago in the
mid-Cretaceous, when the African continent was separated from other major land masses.[25] This idea is consistent with the fossil record of
Xenarthra, which is restricted to South America (following recent consensus that Eurotamandua is not a xenarthran[26]).
However, Afrotheria itself does not have a fossil record restricted to Africa,[27] and appears in fact to have evolved in the continent's isolation.[28] More recent, genomic-scale phylogenies favor the hypothesis that Afrotheria and Xenarthra comprise sister taxa at the base of the placental mammal radiation, suggesting an ancient
Gondwanan clade of placental mammals.[29] A 2021 morphological study also proposed to render
Meridiungulata polyphyletic and recognise most of its clades as part of a group called Sudamericungulata, closely related to hyraxes, while
Litopterna remains a sister taxon to
Perissodactyla.[30]
Relations between the various afrotherian orders are still being studied. On the basis of molecular studies, elephants and manatees appear to be related, and likewise elephant shrews and aardvarks.[31] These findings are compatible with the work of earlier anatomists.[14][15]
A cladogram of Afrotheria based on molecular evidence[15]
Current status and distribution
Many extant members of Afrotheria appear to have a high risk of extinction (perhaps related to the large size of many). Species loss within this already small group would comprise a particularly great loss of genetic and evolutionary diversity. The
IUCN Afrotheria Specialist Group notes that Afrotheria, as currently reconstructed, includes nearly a third of all mammalian orders currently found in Africa and Madagascar, but only 75 of more than 1,200 mammalian species in those areas.[33]
While most extant species assigned to Afrotheria live in Africa, some (such as the Indian elephant and three of the four sirenian species) occur elsewhere; many of these are also endangered. Prior to the
Quaternary extinction event, proboscideans were present on every continent of the world except
Australia and
Antarctica. Hyraxes lived in much of Eurasia as recently as the end of the
Pliocene. The extinct afrotherian orders of
embrithopods and
desmostylians were also once widely distributed. However, the desmostylians have recently been viewed as possible
perissodactyls, rather than afrotheres,[34] although this is still controversial;[28] the taxonomic placement of embrithopods is also not clear.[35]
Classification
Afrotheria is a clade of placental mammals, the stem designation for which is
Eutheria. Based on precedent, some clades are junior synonyms and arguably should be replaced.[36][37]
^
abSánchez-Villagra, Marcelo R.; Narita, Yuichi; Kuratani, Shigeru (2007). "Thoracolumbar vertebral number: The first skeletal synapomorphy for afrotherian mammals". Systematics and Biodiversity. 5 (1): 1–7.
doi:
10.1017/S1477200006002258.
S2CID85675984.
^Le Gros Clark, W.E. & C.F. Sonntag (1926). "A monograph of Orycteropus afer III, the skull, the skeleton of the trunk, and limbs". Proceedings of the Zoological Society of London. 30: 445–485.
^DeJong, W.W.; J.A.M. Leunissen & G.J. Wistow (1993). "Eye lens crystallins and the phylogeny of placental orders: evidence for a Macroscelid–Paenungulate clade?". In F. S. Szalay; M. J. Novacek & M.C. McKenna (eds.). Mammal Phylogeny. New York: Springer Verlag. pp. 5–12.
^
abSimpson, G. G. (1945). "The principles of classification and a classification of mammals". Bulletin of the American Museum of Natural History. 85: 1–350.
^Rose KD, Emry RJ, Gaudin TJ, Storch G (2005). "Xenarthra and Pholidota.". In Rose KD, Archibald JD (eds.). The Rise of Placental Mammals: Origins and Relationships of the Major Extant Clades. Baltimore, MD: Johns Hopkins University Press.